Inkjet printing is characterized by ejecting ink droplets from fine nozzles so as to perform non-contact printing. Companies have carried out R & D for inks for inkjet printing, aiming at making ink droplets finer and making printing speed faster and printing size larger.
As an ink used for inkjet printing (referred to as “ink for inkjet” in the present specification), an aqueous pigment or dye ink is generally used for personal and office uses. Because of high water-absorption of paper fibers, the aqueous pigment or dye ink exhibits high printing density thereon and is low in strike through which is a printing density looked from the backside of prints. On the other hand, since paper is dried under pressure during production, hydrogen bonds between the paper fibers are cut and cause curl when water contacts the paper. This curl is so remarkable as to influence accuracy of positioning of ink droplets and conveyance of paper when cut sheets of paper are used. It is prerequisite to eliminate the curl in order to achieve high speed printing.
As a technique for eliminating the curl, reducing the water content of the ink or removing water from the ink may be employed. In other words, oil based inks can be used to eliminate the curl, which are also suited for high speed printing.
Business printers which adopt a line head type inkjet system are noticed since they have the head fixed so as to produce a large amount of prints at high speed and are available at low price. An oil based pigment ink is usually used for this high speed inkjet printer. However, the ink permeates into the backside of paper after printing, and thus the resulting prints are low in printing density with lots of strike through, compared to printers adopting other systems.
Emulsifying an oil based ink with water to make a water-in-oil (W/O) emulsion ink is one of the methods for solving this problem (refer to Patent Document 1). Although a polyglycerin hydroxyl fatty acid ester with an HLB value of 3-5 is used as an emulsifier in the emulsion ink in Patent Document 1, ejection performance and storage stability are not always sufficient.    [Patent Document 1] JP-A-2006-56931